1. Field of the Invention
The present invention relates generally to the field of automated measuring devices and pertains more specifically to an apparatus for and a method of measuring material thickness using a magnetic technique.
2. Discussion of the Prior Art
There currently exist a number of techniques to measure the thickness of an object. Common techniques include (a) mechanical measurement, (b) ultrasound measurement, and (c) magnetic measurement. A mechanical measurement is usually performed using a mechanical instrument such as a pair of calipers. While the mechanical measurement can give accurate results, it is awkward, tedious, and time consuming to measure the thickness of large objects or objects with a complicated geometry. While ultrasound measurements can measure the thickness of many materials, it is difficult to use ultrasound to measure the thickness of materials which are inhomogeneous, such as composite materials, or those which have internal structures that can scatter ultrasound, such as honeycomb structures.
Magnetic thickness measurement can be divided into different categories. One type of magnetic measurement is the DC magnetic measurement. In a DC magnetic thickness measurement, a magnet is placed on one surface of the sample and the resultant magnetic field is measured at the opposite surface of the sample, using a magnetic field sensor such as a Hall sensor. Since the magnetic field falls with distance from the magnet, the thickness of the sample can be deduced from the magnitude of the measured magnetic field. Alternatively, both the magnet and the magnetic sensor are placed on the same surface of the sample, and the sample thickness is determined by measuring the response from a magnetically permeable target placed on the opposite surface of the sample. One example of a DC magnetic thickness measurement is described in U.S. Pat. No. 5,539,675 to Carroll Sr. et al. This invention preferably employs a permanent magnet to automatically measure the wall thickness of an article such as a bottle, using a set of positioning motors. The magnet generates a DC field which induces a response from a steel ball target positioned opposite to the magnet on the other side of the bottle wall. The response from the target depends on the wall thickness, which can then be determined. One disadvantage of this method is that the DC magnetic measurement is subject to errors due to the Earth""s magnetic field and the intense low-frequency magnetic background noise present in many industrial environments. In addition, since the magnetic field is produced by a permanent magnet, the magnitude of the field changes with temperature. These errors limit the accuracy and precision of the thickness measurement, especially for thick samples.
A second type of magnetic thickness measurement uses the response of a magnetic target to an AC, or oscillatory, magnetic field. This technique has been applied to measure the thickness of nonmagnetic films, coatings, or sheets placed on top of a magnetic substrate that acts as the target. An example of this method is described in U.S. Pat. No. 5,770,949 to Sgro. Here a coil is placed on one side of the sample and a magnetic target is placed on the other side. An AC magnetic field is produced by passing an oscillatory current through the coil. This AC magnetic field is distorted by the magnetic permeability of the target and this field distortion changes the inductance of the coil. The thickness of the sample is then determined from the magnitude of the change in the coil inductance. Nix (U.S. Pat. No. 5,467,014) has also described a similar technique, known as the xe2x80x9cmagnetoinductivexe2x80x9d technique, in which the magnetic response of the sample changes coil inductance.
Another variation of the AC magnetic thickness measurement uses the eddy current response from a metallic target. Here a coil generates an oscillatory, or AC, magnetic field that induces eddy currents in a metallic target on the opposite surface of the sample. The thickness of the sample is then determined by measuring the magnetic field produced by this eddy-current response. Most commonly, this technique has been used to measure the thickness of a film or coating on a conducting substrate. An example of this technique is described in U.S. Pat. No. 5,963,031 to de Halleux et al. Here an excitation coil supplies an AC magnetic field to a ferromagnetic, electrically conducting material covered with a non-ferromagnetic, electrically conducting coating. The response from the material and coating consists of a combination of the eddy current response from the magnetic material and that from the coating. This response is measured with a receive coil and used to determine the coating thickness.
Nix (U.S. Pat. No. 5,467,014) describes two probes as part of the same device. The first being a magnetoinductive probe to measure the thickness of film coatings on magnetic or ferromagnetic substrates and the second being an eddy current probe to measure the thickness of film coating on conducting, but non-magnetic, substrates. The two probes are necessary since they operate at different AC frequencies. Rather than have to rely on two probes, one would prefer to utilize the same probe for measurements at all frequencies.
In contrast with the DC-field method, these AC-field methods are not subject to errors due to the Earth""s magnetic field. One disadvantage of these methods is that, when the sample itself is electrically conductive, the magnetic measurements will be affected by eddy currents induced in the sample. As described below, one would prefer to reduce this effect by reducing the frequency of the AC magnetic field.
Another disadvantage, shared by all of the methods described above, is that the magnetic sensor or detection coil is subjected to the applied AC or DC field, as well as the field due to the response of the target. In most cases, the target response is much smaller than the applied field, so that small drifts in the applied field, or in the gain of the sensor system, can produce large errors in measuring the target response.
The magnetic measurements described above are generally used to measure the thickness of nonmagnetic materials since magnetically permeable materials distort the applied magnetic field. However, in principle, one could calibrate the response from a section of a given magnetically permeable material to a DC or a very low frequency AC magnetic field, and use this information to determine the thickness of other sections of the same material.
A definite need exists for a system having an ability to provide rapid, accurate thickness measurement of materials. In particular, a need exists for a system which is capable of providing these measurements in a non-destructive manner. Ideally, such a system would have a lower cost and a higher sensitivity than conventional systems. A primary purpose of the present invention is to improve the capability of the magnetic thickness measurement technique using an accurate and versatile thickness measurement device.
An apparatus for and a method of measuring material thickness with magnetics is disclosed. The thickness monitoring system includes a thickness monitor, a probe, and a target. In a preferred embodiment, the probe is positioned on one side of an article for which the thickness is to be determined. The target is positioned on the opposite side of the article from the probe. The probe includes an excitation coil, a field compensation coil, and a magnetic sensor. The method includes energizing the excitation coil to excite a response from the target, compensating for the effect of the excitation coil on the magnetic sensor, measuring the response of the target with the magnetic sensor, and determining the thickness of the article from the measured response. The preferred mode of energizing the excitation coil is with an AC waveform; however DC, multi-frequency AC, or a combination of AC and DC waveforms may be used. Other preferred embodiments include:
(1) Omitting the excitation and field compensating coils and exciting the target by passing a current of AC, DC, or both through the target to generate a magnetic field at the sensor. In this preferred embodiment, the target is a strip of conductor or a conducting coil. The magnetic field generated by passing a current through the target is measured by the magnetic sensor on the opposite side of the sample. The sample thickness is then determined from the strength of the magnetic field at the sensor.
(2) Omitting the target and determining the sample thickness from the magnetic response of the sample itself.
(3) Using two or more excitation coils producing magnetic fields of differing spatial distribution.
(4) Using two or more magnetic sensors to detect the spatial distribution of the magnetic field generated by the target.